1. Field of the Invention
The present invention relates to a current/voltage control apparatus for an elevator system, and in particular to an improved current/voltage control apparatus for an elevator system which is capable of preventing an over-current and over-voltage from being transferred to an inverter.
2. Description of the Background Art
As the technology of a high-power semiconductor device is advanced, the motor type for the elevator system is changed from the direct current motor to the induction motor. For controlling the induction motor, recently a vector control method is increasingly used based on a VVVF (Variable Voltage Variable Frequency). In particular, the input current of the induction motor is separated into a magnetic component current and a torque component current for vector-controlling the induction motor, so that an instantaneous torque control of the induction motor is made possible.
In order to VVVF-control the induction motor, there are provided a converter for converting a three-phase alternating current voltage into a direct current voltage and an inverter for converting the direct current voltage into an alternating current voltage of a variable voltage type and variable frequency type. The instantaneous torque control of the induction motor is implemented by controlling the switching operation of the inverter. Therefore, the current/voltage control apparatus is designed to prevent an abnormal operation of the inverter.
FIG. 1 illustrates the inner construction of the conventional current/voltage control apparatus for an elevator system.
As shown therein, the conventional elevator system includes a converter 1 for converting a three-phase alternating current voltage AC into a direct current voltage by rectifying the same, an inverter 2 for converting the thusly converted direct current voltage into an alternating current voltage of a variable voltage and frequency type, an induction motor 4 for generating force for running an elevator car 8 using the thusly converted alternating current voltage, current sensors 3A through 3C for sensing three-phase current flowing in the induction motor 4, a sheave 5, rope 6 and balance weight 7 for receiving a force from the induction motor 4 and running the elevator car 8 along a running path formed through each floor, a velocity detector 9 for detecting an actual rotational frequency .omega..sub.r of the induction motor 4, an over current detector 10 for comparing the current of each phase sensed by the current sensors 3A through 3C with a previously set limit current value and generating a driving stop signal of the inverter 2 when the current thereof exceeds the limit current value as a result of the comparison, current amplifiers 11A through 11C for amplifying the current of each phase sensed by the current sensors 3A through 3C to a predetermined level, respectively, a current transformer 12 for the three phase currents (i.sub.a, i.sub.b, i.sub.c) from the current amplifiers 11A through 11C into a torque component (Q-axis) current (i.sub.q), a magnetic component (D-axis) current (i.sub.d), a velocity instruction generator 13 for generating a velocity instruction (W*) corresponding to the operational instruction (D*), a subtractor 14 for computing a difference of an actual rotational frequency (.omega..sub.r) between the velocity instruction (W*) and the induction motor 4, a velocity controller 15 for generating a torque component current instruction (i.sub.q *) which is proportional to the above difference, a magnetic component current instruction generator 16 for generating a magnetic component current instruction (i.sub.d *) corresponding to the operational instruction (D*) , a subtractor 17 for computing a difference with respect to the torque component current (i.sub.q) from the current transformer 12, a subtractor 18 for computing a difference between the magnetic flux component current instruction (i.sub.d *) outputted from the magnetic flux component current instruction generator 16 and the magnetic flux component current (i.sub.d) outputted from the current transformer 12, Q-axis and D-axis current controllers 19A and 19B for generating Q-axis and D-axis voltage instructions (V.sub.q, V.sub.d) for controlling the Q-axis current and d-axis current from the induction motor 4 based on the output currents from the subtractors 17 and 18, a slip computation unit 20 for computing a slip frequency .omega..sub.s of the induction motor 4 based on the torque component current instruction (i.sub.q *) , an adder 21 for adding the rotational frequency .omega..sub.r detected by the velocity detector 9 and the slip frequency .omega..sub.s computed by the slip computation unit 20 and outputting an operational frequency .omega..sub.e of the induction motor 4, a voltage transformer 22 for converting the operational frequency .omega..sub.e and the Q-axis and D-axis voltage instructions V.sub.q, V.sub.d into three phase voltage instructions V.sub.a, V.sub.b, V.sub.c, a voltage limiter 23 for limiting the output ranges of the three phase voltage instructions V.sub.a, V.sub.b, V.sub.c, and an inverter driving unit 24 for generating a pulse modulation signal corresponding to the inverter driving control signal from the voltage limiter 23 or the over current detector 10 and driving the inverter 2.
The operation of the conventional elevator system will now be explained with reference to FIGS. 1 through 4.
First, when a three phase alternating current (AC) voltage is inputted, the three phase alternating current voltage is converted into a direct current voltage by the converter 1 and is smoothed by a condenser C and is supplied to the inverter 2. The inverter 2 converts the thusly inputted direct current voltage into an alternating current voltage of a variable voltage type and variable frequency type and then is supplied to the induction motor 4.
In addition, when the velocity detector 9 detects an actual rotational frequency .omega..sub.r of the induction motor 4, and the velocity instruction generator 12 generates a velocity instruction W* corresponding to the operational instruction D*, the subtractor 14 computes a difference between the velocity instruction W* and the rotational frequency .omega..sub.r. The velocity controller generates a torque component current instruction (i.sub.q *) which is proportional to the difference, and the magnetic flux component current instruction generator 16 generates a magnetic flux component current instruction (i.sub.d *) in accordance with the operational instruction D*.
FIGS. 2A through 2C are wave form diagrams of the velocity instruction W*, the torque component current instruction (i.sub.q *) and the magnetic flux component current instruction (i.sub.d *).
When the velocity instruction W*, as shown in FIG. 2A, is generated by the velocity instruction generator 13, the velocity controller 15 generates the torque component current instruction (i.sub.q *), as shown in FIG. 2B, which is proportional to the difference between the velocity instruction W* and the rotational frequency .omega..sub.r. The magnetic flux component current instruction generator 16 generates the current instruction (i.sub.d *) having a predetermined size, as shown in FIG. 2C, while the operational instruction D* is in the RUN state.
In addition, the three phase current flowing in the induction motor 4 is detected by the current sensors 3A through 3C, and the thusly detected three phase current is amplified to a predetermined level by the current amplifiers 11A through 11C.
At this time, the over current detector 10 compares the current value detected by the current sensors 3A through 3C with the set limit current value I.sub.-- limit. As a result of the comparison, if the three phase current value exceeds the same, the inverter driving stop signal is generated.
FIGS. 3A and 3B illustrate wave form diagrams for explaining an operational timing of the over current detector 10.
At the point P where the current value detected by the current sensors 3A through 3C exceeds a limit current value I.sub.-- limit as shown in FIG. 3A, the over current detector 10 generates an inverter driving stop signal as shown in FIG. 3B.
In addition, the three phase currents i.sub.a, i.sub.b, and i.sub.c, amplified by the current amplifiers 11A through 11C are converted into the Q-axis and D-axis currents i.sub.q, i.sub.d by the current transformer 12 based on the Equations 1 through 3. EQU i.sub.a =Icos (.omega..sub.e -.theta.) EQU i.sub.b =Icos (.omega..sub.e -2.pi./3-.theta.) EQU i.sub.c =Icos (.omega..sub.e -2.pi./3+.theta.) . . . Equation 1 EQU i.sub..alpha. =(i.sub.c -i.sub.b).sqroot.3=-Isin (.omega..sub.e -.theta.) EQU i.sub..beta. =i.sub.a =Icos (.omega..sub.e -.theta.) . . . Equation 2 EQU i.sub.d =cos (.omega..sub.e t).times.i.sub..alpha. +sin (.omega..sub.e t).times.i.sub..beta. =Isin (.theta.) EQU i.sub.q =-sin (.omega..sub.e t).times.i.sub..alpha. +cos (.omega..sub.e t).times.i.sub..beta. =Icos (.theta.) . . . Equation 3
where .omega..sub.e denotes the operational frequency, and .theta. denotes the phase angle.
As seen in Equation 3, the Q-axis and D-axis currents i.sub.q and i.sub.d are inputted into the subtractors 17 and 18, respectively, and are compared with the torque component current instruction (i.sub.q *) and the magnetic flux component current instruction (i.sub.d *) outputted from the velocity controller 15 and the magnetic flux component current instruction generator 16. When a difference value corresponding to the comparison is outputted, the Q-axis current controller 19A and D-axis current controller 19B generate the Q-axis and D-axis voltage instructions V.sub.q and V.sub.d.
In addition, when the slip computation unit 20 outputs a slip frequency .omega..sub.s which is proportional to the torque component current instruction (i.sub.q *) outputted from the velocity controller 15, the adder 21 adds the slip frequency .omega..sub.s and the rotational frequency .omega..sub.r of the induction motor 4, thus outputting an operational frequency .omega..sub.e.
In addition, the voltage transformer 22 outputs the three phase voltage instructions V.sub.a, V.sub.b, and V.sub.c based on the following Equations 4 and 5 using the operational frequency .omega..sub.e outputted from the adder 21 and the Q-axis and d-axis voltage instructions V.sub.q and V.sub.d generated by the Q-axis and D-axis current controllers 19A and 19B. EQU V.sub..alpha. =-V.sub.q .times.sin (.omega..sub.e t)+V.sub.d .times.cos (.omega..sub.e t) EQU V.sub..beta. =+V.sub.q .times.cos (.omega..sub.e t)+V.sub.d .times.sin (.omega..sub.e t) . . . Equation 4 EQU V.sub.a =V.sub..beta. ##EQU1##
The thusly computed three phase voltage instructions V.sub.a, V.sub.b and V.sub.c are inputted into the voltage limiter 23 and compared with the limit voltage value V.sub.-- limit. Only the voltage is below the limit voltage value V-limit is applied to the inverter driving unit 24, so that the over voltage is not applied to the inverter 2 and the induction motor 4.
FIGS. 4A and 4B illustrate the wave form diagrams of voltage signals applied to the induction motor 4.
When the velocity instruction W*, as shown in FIG. 4A, occurs, the applying voltage of the induction motor 4 which is proportional to the thusly occurring velocity instruction W* is outputted as shown in FIG. 4B. Here, V-rate denotes a rated voltage, and V-limit denotes a limit voltage value.
As shown in FIG. 4B, the applying voltage which is limited below the limit voltage value V.sub.-- limit is inputted into the inverter driving unit 24, and the inverter driving unit 24 outputs a pulse-modulated signal to the inverter 2 when the operational instruction D* is in a RUN state, namely, it is activated, thus switching and internal power transistor, so that it is possible to control the velocity and current of the induction motor.
In addition, the rotational force of the induction motor 4 is transferred to the car 8 through the sheave 5 and rope 6, so that the car 8 is moved to a destination floor along the running path.
In the conventional current/voltage control apparatus for an elevator system, the over current detector 12 and the voltage limiter 23 are provided for preventing an over current and voltage from being transferred to the inverter 2.
As shown in FIG. 4, the limit voltage value V.sub.-- limit of the voltage limiter 23 always maintains a constant value. However, the applying voltage applied to the induction motor 4 is always proportional to the velocity of the induction motor 4.
Therefore, when the induction motor 4 operates at a low velocity, the Q-axis and D-axis currents are controlled by a voltage which is lower than the rated voltage V.sub.-- rate, so that it is impossible to obtain a desired prevention operation with respect to the system.
In addition, if the three phase parameters of the induction motor 4 are parallel, the three phase currents flowing in the induction motor 4 become parallel based on Equation 1, so that the sum of the three phase currents, namely, the zero phase component current becomes 0. If the three phase parameters are not parallel due to a predetermined factor, since the three phase currents flowing in the induction motor 4 are not parallel, the sum of the three currents does not become zero.
In this case, the current flowing in the induction motor 4 may become smaller than the limit current value I.sub.-- limit of the over current detector 10, or may become larger than the rated current of the induction motor 4. since the conventional over current detector 10 is not capable of detecting the above-described problem, the elevator system is continuously operated irrespective of the above-described problems, so that more serious problem may occur.
In addition, in the current control circuit if the current value is not detected due to a failure of the current sensors 3A through 3C, or the short circuit of the sensor output cable, a large voltage instruction occurs in the Q-axis and D-axis current controllers 19A and 19B during a low velocity operation. Therefore, an over current flows in the inverter and induction motor. When this over current is not detected, the inverter and motor may be damaged.